Ad. Jassby et al., THE DESIGN OF SAMPLING TRANSECTS FOR CHARACTERIZING WATER-QUALITY IN ESTUARIES, Estuarine, coastal and shelf science, 45(3), 1997, pp. 285-302
The high spatial variability of estuaries poses a challenge for charac
terizing estuarine water quality. This problem was examined by conduct
ing monthly high-resolution transects for several water quality variab
les (chlorophyll a, suspended particulate matter and salinity) in San
Francisco Bay (California, U.S.A.). Using these data, six different wa
ys of choosing station locations along a transect, in order to estimat
e mean conditio:ns, were compared. In addition, Il approaches to estim
ating the variance of the transect mean when stations are equally spac
ed were compared, and the relationship between variance of the estimat
ed transect mean and number of stations was determined. The results pr
ovide guidelines for sampling along the axis of an estuary: (I) choose
as many equally-spaced stations as practical; (2) estimate the varian
ce of the mean (y) over bar by var (y) over bar=(1/10n(2))Sigma(j=2)(n
) (y(j)-y(j-1))(2), where y(1),..., y(n) are the measurements at the n
stations; and (3) attain the desired precision by adjusting the numbe
r of stations according to var (y) over bar proportional to 1/n(2). Th
e inverse power of 2 in the last step is a consequence of the underlyi
ng spatial correlation structure in San Francisco Bay; more studies of
spatial structure at other estuaries are needed to determine the gene
rality of this relationship. (C) 1997 Academic Press Limited.